Technical Abstract:
Bumblebees and hawkmoths are the two major pollinators of the blue columbine, Aquilegia coerulea. Hawkmoths collect nectar and visit flowers mostly at dusk while bumblebees collect pollen and visit flowers during the day. Pollinator abundance varies among populations and among years within some of the populations. Mixed mating, where both selfing and outcrossing occurs in a population, is common in the blue columbine. Outcrossing rate, estimated using genetic markers in progeny arrays, ranges between 0.46 and 0.93 in different populations. We used three different approaches to determine whether hawkmoths increased the outcrossing rate relative to bumblebees in this plant species. First, we quantified the relationship between outcrossing rate and hawkmoth abundance among populations. Second, we compared the outcrossing rate over years within a population where hawkmoth abundance varied among years. Lastly, we contrasted the outcrossing rate of groups of plants visited by only day or only dusk/night pollinators. Wihin a population, plants were either caged during the day to prevent bumblebee visits, or caged at dusk to prevent hawkmoth visits. A group of plants served as the control and was not caged. In all three approaches we expected outcrossing rate to increase with hawkmoth abundance. Pollinator observations identifed behavioral differences between pollinators that helped explain the observed differences in outcrossing rate.